Peadar wrote:I'm trying to read some irish news lately, some international articles keep using the word "Breatimeachta" which I can't find in every irish dictionary nor irish Wikipedia(Nor in Wikitionary FYI)

As an aside, you might notice that both of these words violate the rule of caol le caol agus leathan le leathan, i.e. Breatimeacht, drochimpleacht. That almost always indicates that you're dealing with a compound word (or a poor speller).

"Richmond is a real scholar; Owen just learns languages because he can't bear not to know what other people are saying."--Margaret Lattimore on her two sons

Does this mean that the lenition of the possessed noun is not just a phonetic inflection, but a grammatical one?

Lenition is characteristic of the vocative. The vocative particle a lenits, but is often dropped in speech. Cf. the title of another ballad, "Mháirín Óg Ní Cheallaigh". Máirín is lenited here because it represents direct address. (The title is alternatively given as "A Mháirín Óg Ní Cheallaigh", with a leading vocative particle that is not pronounced.)

That said, can I ask where you found this version? The versions I'm familiar with all have "'Sé mo Shéasar, gile mear", without lenition.

"Richmond is a real scholar; Owen just learns languages because he can't bear not to know what other people are saying."--Margaret Lattimore on her two sons